Minimizing non-linearity errors

ABSTRACT

A system and method for minimizing non-linearity errors induced in output drive voltage of a transmitter circuit due to on-chip process, voltage, and temperature (PVT) variations. The system including an oscillator for converting an input reference bias voltage into a clock output signal, where the input reference bias voltage varies in response to PVT variations. Also included is a counter for counting the clock output signal and generating a count value corresponding to the clock output of the oscillator. A comparison module operatively coupled to the counter compares the count value with a pre-simulated count value to generate an error signal. Based on the error signal generated by the comparison module, a correction logic adjusts an output drive signal of the transmitter circuit making it immune to PVT variations.

TRADEMARKS

IBM® is a registered trademark of International Business MachinesCorporation, Armonk, N.Y., U.S.A. Other names used herein may beregistered trademarks, trademarks or product names of InternationalBusiness Machines Corporation or other companies.

TECHNICAL FIELD

Embodiments of the invention relate generally to performance parametersof integrated circuits (ICs), and more specifically, to on-chip process,voltage, and temperature (PVT) variations in a transmitter circuit.

BACKGROUND OF THE INVENTION

Integrated circuit (IC) feature sizes are shrinking steadily into thesub-100 nm range, resulting in increased sensitivity of an IC to on-chipprocess, voltage, and temperature (PVT) variations. This, in turn,hampers IC performance as well as IC fabrication yields. In general, ICsare fabricated and expected to operate under nominal PVT corner;however, after or during fabrication the PVT parameters inevitably driftfrom their nominal values. These PVT variations cause various criticalcircuit performance parameters to deviate from their expected nominalbehavior, such as delay, linearity, input/output impedances, staticpower dissipation, and other performance parameters well known in theart.

On-chip PVT variations can cause fabricated chips to fall short of theirperformance requirements and significantly lower the overall yield ofthe chips. Under nominal PVT conditions, a typical IC, such as aSerializer/Deserializer (SerDes) transmitter circuit, exhibits a linearoutput voltage response. After fabrication however, PVT variationsinduce non-linearity errors in the output voltage response of the IC.Two main contributors to on-chip variability are changes in processparameters, and changes in operating temperatures. Process parametervariations can occur due to proximity effects in photolithography,non-uniform conditions during deposition, random dopant fluctuations,etc., resulting in fluctuations in parameters such as channel length andwidth, oxide thickness, dopant concentrations, and threshold voltage.Changes in operating temperatures stem from variations in heatdissipation, leading to a decrease in charge carrier mobility, which inturn causes a direct decrease in current and circuit speed reduction.Moreover, an increase in the operating temperature can also lead toincrease in leakage current.

As a result of PVT variations, an IC may require tuning afterfabrication. While the effects of process parameters require one-timecompensation immediately after the transmitter is fabricated, thermalvariations are dependent on the operating environment and as a resultthe transmitter require an adaptive on-chip circuit that monitors thePVT variations and performs a runtime compensation to minimize thevariations. Thus, existing measures fail to address the full scope ofthe issues raised by PVT variations.

SUMMARY OF THE INVENTION

In one embodiment, an adaptive circuit for minimizing non-linearityerrors induced in output drive voltage of a transmitter circuit due toon-chip process, voltage and temperature (PVT) variations. The adaptivecircuit includes an oscillator, a counter, a comparison module and acorrection logic, operatively coupled. The oscillator converts an inputreference bias voltage into a clock output. The input reference biasvoltage varies in response to the PVT variations and frequency of theclock output varies according to changes in the input reference biasvoltage. The counter receives input from the oscillator and generates acount value corresponding to the clock output of the oscillator. Thecomparison module is configured to compare the count value with apre-simulated count value to generate an error signal. Using the errorsignal, the correction logic module adjusts output drive signal, whichcompensates the PVT variations.

In another embodiment, a method for minimizing non-linearity errorsinduced in output drive voltage of a transmitter circuit due to process,voltage, and temperature (PVT) variations. The method includesgenerating a clock output signal from an input reference bias voltage,wherein frequency of the clock output signal varies in response to thePVT variations. The method also includes counting the clock outputsignal to generate a count value and comparing the count value with apre-simulated count value to compute an error signal. Further, basedupon the generated error signal the method includes adjusting outputdrive signal, which compensates the PVT variations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an embodiment of a transmitteremployed in communication systems.

FIG. 2 illustrates a block diagram of an embodiment of an adaptivecircuit for minimizing effects of on-chip PVT variations on theperformance parameters of a transmitter.

FIG. 3 illustrates an embodiment of a PVT sensing logic for sensingnon-linearity errors induced in output voltage of a transmitter.

FIG. 4 illustrates an embodiment of a PVT correction logic forcorrecting non-linearity errors induced in output voltage of atransmitter.

FIG. 5 is a flowchart of a method for minimizing effects of on-chip PVTvariations on performance parameters of a transmitter.

FIG. 6 is a flowchart depicting simulation phase of a method forminimizing non-linearity errors in a transmitter.

FIG. 7 is a flowchart depicting on-chip calibration phase of the methodfor minimizing non-linearity errors in a transmitter.

FIG. 8 is a graph depicting PVT drift introduced linearity errors intransmitter output

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is made with reference to thefigures. Preferred embodiments are described to illustrate theinvention, not to limit its scope, which is defined by the claims. Thoseof ordinary skill in the art will recognize a number of equivalentvariations on the description that follows.

Referring now to the drawings, FIG. 1 sets out a block diagram of anembodiment of a transmitter 100 employed in communication systems. Thetransmitter 100 is included in a high speed serial linkSerializer/Deserializer (SerDes) core. A conventional SerDes transmitteris an integrated circuit (IC) employed in high-speed communicationdevices for converting parallel data to serial data. The transmitter 100includes a serializer 102, a predriver 104, a reference currentgenerator 106, a tap coefficient module 108, a current multiplier 110,an output (O/P) driver 112, and a channel 114. These elements are allconventional and all perform functions known and understood by those inthe art; therefore, these components will not be discussed further here.The terms “transmitter” and “SerDes transmitter” are usedinterchangeably in the following discussion. The invention is discussedherein with reference to a SerDes transmitter, but those skilled in theart will appreciate that the invention may be applicable to any IC whichis affected by process, voltage, and temperature (PVT) variations.

In general, output voltage of a SerDes transmitter exhibits a linearcurve with respect to input parameters. In FIG. 1, differential voltageV_(OD) of output serial data of the transmitter 100 varies linearly as afunction of its input tap coefficient settings (TXCO). On-chip PVTvariations, however, induce non-linearity errors in the output voltageresponse of the transmitter 100. The terms “PVT variations” and “on-chipPVT variations” are interchangeably used in the following sections.Typically, PVT parameters vary from best case performance corner toworst case performance corner, as known to those skilled in the art, andthese PVT corners are industry set standards. For each IC, a nominalcorner exists that corresponds to an optimized PVT corner yielding anoptimized performance by the IC. The transmitter 100 exhibits a linearoutput voltage response at a nominal PVT corner. In general, PVTvariations are represented using a Gaussian distribution curve, wherethe nominal PVT corner exists at zero skew. A skew of +3 or +6 measuredfrom the nominal corner indicates the best case performance corner and−3 or −6 skew from the nominal corner indicates the worst caseperformance corner, for example.

The transmitter 100 includes an adaptive circuit 116 that sensesnon-linearity errors in the output voltage and acts to minimize thoseerrors. This circuit allows the transmitter 100 to produce a linearoutput voltage for any PVT condition. The adaptive circuit 116 will bediscussed in detail in connection with FIGS. 2-7, but in brief theadaptive circuit 116 receives input V_(BG) from the reference currentgenerator 106, and it dynamically adjusts an output drive signalsupplied to the O/P driver 112, and the output drive signal compensatesfor PVT variations.

The adaptive circuit 116 can be implemented in a number of forms, andthe following sections set out a number of examples showing specificembodiments for such a circuit. It will be understood that the followingdescriptions are not intended to be exhaustive, and those of skill inthe art will recognize other variations possible within the ambit of theclaims, which solely define the invention.

A first such implementation is seen in FIG. 2, a block diagram of anadaptive circuit 200 for minimizing effects of on-chip PVT variations onthe performance parameters of a transmitter. The adaptive circuit 200may be embodied within a circuit design of a conventional transmitter,such as an ASIC of a transmitter. The adaptive circuit 200 includes anoscillator 202, a counter 204, a pre-simulated count value 206, acomparison module 208, and a correction logic 210. These elements areshown as functional blocks, which can be implemented employing any ofthe components or designs in general use in the art. For example, thosein the art will recognize the oscillator 202 could conveniently befabricated as a voltage controlled oscillator (VCO). Similarly, thecounter 204 could employ a conventional n-bit counter and latch circuit,or any other suitable device.

The adaptive circuit 200 receives a reference bias voltage V_(CTRL) andadjusts an output drive signal of the transmitter compensating foron-chip PVT variations. Initially, the oscillator 202 receives V_(CTRL)and converts it into a clock output signal. It will be appreciated bythose skilled in the art that the value of V_(CTRL) varies in responseto the PVT variations, and thus the frequency of the clock output signalvaries in response to variations in the value of V_(CTRL). The counter204 receives the clock output signal and generates a count valueproportional to the clock output signal of the oscillator 202. Next, thecomparison module 208 compares the count value with the pre-simulatedcount value 206 to generate an error signal. The pre-simulated countvalue 206 refers to a count value computed for a nominal PVT cornerduring transmitter design. Typically, pre-simulated count values arederived through simulation, employing computer-based design tools suchas SPECTRE (produced by Cadence Corporation), HSPICE (by SynopsisCorporation) and the like. Such tools are well known to those in the artand are typically used to determine the performance and functionality ofan IC design before committing that design to actual fabrication.

Using the error signal generated in the comparison module 208, thecorrection logic 210 adjusts the output drive signal, which compensatesfor non-linearity errors introduced in the transmitter due to the PVTvariations. Some embodiments include changing the values of output drivecurrent and output drive voltage depending on the adjustments in theoutput drive signal. Those skilled in the art will appreciate that an ICmay employ more than one adaptive circuit for sensing and correctingon-chip PVT variations.

The adaptive circuit 200 performs two major functions: sensing PVTvariations and minimizing the effect of those variations. Thesefunctions can be reflected in adaptive circuit design, producing theadaptive circuit 200 divided into two logic modules, a PVT sensing logicand a PVT correction logic. The PVT sensing logic will be described inthe following section in connection with FIG. 3, and the PVT correctionlogic will be described in connection with FIG. 4.

FIG. 3 illustrates an embodiment of a PVT sensing logic 300, for sensingnon-linearity errors induced in an output voltage of a transmitter dueto PVT variations. The transmitter includes a conventional bias circuit302 that converts voltage into current. The bias circuit 302 is wellknown to those skilled in the art and will not be explained in detailhere. The bias circuit 302 receives a standard reference bias voltage,referred to as band gap voltage V_(BG), as input, which is convertedinto its equivalent reference current value using an operationalamplifier in closed loop, as shown. The standard value of V_(BG) isbased on the nominal PVT corner, and the value of V_(BG) varies with PVTparameters. In an exemplary embodiment, PVT variations affect resistanceof a resistor 303 and in response the output voltage of the bias circuit302 varies. The bias circuit 302 generates a reference bias voltageV_(CTRL), which is supplied as an input control voltage to a VCO 304.

The reference bias voltage V_(CTRL) is then converted into a count valueby the VCO 304 and a counter 306. The VCO 304 first converts V_(CTRL)into a clock output signal, whose frequency varies in response tochanges in V_(CTRL) and then the VCO output is provided as input to thecounter 306, which generates a count value corresponding to the PVTparameters. The count value generated by the counter 306 varies with theVCO output frequency, corresponding to variations in the PVT parameters.The counter 306 shown in FIG. 3 can be an n-bit counter, including alatch circuit 307, where the latch 307 is preset to an initial statecount value corresponding to the nominal PVT corner. The operation ofthe counter 306 is explained in detail in the following sections.

The PVT sensing logic 300 also includes a clock gate 308, which providesa gated clock signal to the counter 306. The gated clock signal enablesthe counter 306 throughout the operation of the transmitter afterfabrication, and this gated clock signal is computed during simulation.The counter 306 counts the clock output signal until the gated clocksignal is in high state, and it generates a count value when the gatedclock signal goes low. The count value generated by the counter 306 isprovided as an input to a comparison module 309 for computing an errorsignal. The comparison module 309 includes a look-up table 310, asubtract module 312, and a set of multiplexers, an increment multiplexer(INC MUX) 314 and a decrement multiplexer (DEC MUX) 316.

The IC design process includes simulation of IC characteristics andoperating parameters, as noted above. Simulation of the PVT sensinglogic 300 includes computing a reference count value corresponding tothe nominal PVT corner, which can be accomplished by computing pulsewidth of the gated clock signal supplied by the clock gate 308 to thecounter 306, using the VCO frequency at the nominal PVT corner. Thetransmitter simulation operates under the nominal PVT corner value togenerate a count value referred to as a reference count value, which isstored in the look-up table 310. Further, reference count values mayalso be computed for each of the different PVT corners. It will beappreciated by a person skilled in the art that any data structure knownin the art may be employed for storing the reference count valuesgenerated for all PVT corners. The operation of the adaptive circuitduring simulation will be discussed in detail in connection with FIG. 6.

During on-chip operations, i.e., after fabrication, the PVT sensinglogic 300 senses deviations in the count value computed by the counter306 from the reference count value, where deviations correspond to anyon-chip PVT variations. Initially, the counter 306 is preset to apredetermined state corresponding to the nominal PVT corner. The clockgate 308 provides a standard gated signal to the counter 306 (that is,the same output employed in the simulation). The counter 306 thengenerates a count value relating to the PVT condition at any point oftime. The latch 307 is used as a memory element to store the count valuecorresponding to the falling edge of the gated clock signal. Thislatched count value generated by the counter 306 can be used to computethe amount of error introduced due to PVT drift. Next, the subtractmodule 312 subtracts the reference count value corresponding to thenominal PVT corner, computed during simulation, from the count value togenerate an error count value with a sign bit value.

If the output of the counter 306 is less than the reference count valuestored in the look-up table 310, the subtract module 312 generates anerror count value and a negative sign bit value. A negative sign bitvalue indicates that at this point of time the transmitter is operatingunder PVT parameters that are less than the nominal PVT corner values.The error count value together with the negative sign bit value isprovided to the increment multiplexer 314, which generates an n-bitincrement error signal, referred to as INC_CTRL<N:0>.

If the output of the counter 306 is, however, greater than the referencecount value in the look-up table 310, the subtract module 312 thengenerates an error count value with a null sign bit value. A null signbit value indicates that the error count value is positive, meaning thatthat the transmitter is operating under PVT parameters that are greaterthan the nominal PVT corner values. The error count value is provided asinput to the decrement multiplexer 316, which in turn creates an n-bitdecrement error signal, referred to as DEC_CTRL<N:0>.

Thus, the PVT sensing logic 300 is configured to dynamically sense allnon-linearity errors induced in output voltage of a transmitter due toon-chip PVT variations and generates either an increment error signal ora decrement error signal corresponding to the non-linearity error.

FIG. 4 illustrates an embodiment of a PVT correction logic 400 forcorrecting non-linearity errors sensed employing the PVT sensing logic300 in a transmitter. The PVT correction logic 400 receives either anincrement error signal or a decrement error signal from the PVT sensinglogic 300. The PVT correction logic 400 includes a set of p-type metaloxide semiconductor (PMOS) transistors, 404, 406, 408 and a set ofn-type metal oxide semiconductor (NMOS) transistors 410, 412 and 414,where each pair of PMOS transistors and NMOS transistors is coupled inseries to form fingers, coupled in parallel, as shown in FIG. 4. EachPMOS transistor 404, 406 and 408 receives one-bit signal from an n-bitincrement error signal, INC_CTRL<N:0>, while each NMOS transistor410,412 and 414 receives one-bit signal from an n-bit decrement errorsignal, DEC_CTRL<N:0>. FIG. 4 depicts only a few of the PMOS and NMOStransistors for simplicity of discussion, however, a real scenario wouldinclude a large number of PMOS and NMOS transistors.

If the PVT correction logic 400 receives an n-bit increment error signalINC_CTRL<N:0> from the PVT sensing logic 300, then each bit of theincrement error signal is connected to one of the PMOS transistors 404,406, and 408 and some of the PMOS transistors 404, 406, and 408 areselectively turned ON depending on the value of bits of theINC_CTRL<N:0>. The turned ON PMOS transistors increase the output drivesignal, which is supplied to an O/P driver of the transmitter. Theoutput drive signal will change the value of the output drive voltageand an output drive current of the transmitter. The embodiment shownincreases the total output drive current flowing to the O/P driver ofthe transmitter using the increment error signal and thereby compensatesfor all the non-linearity errors induced due to PVT variations.

If the PVT correction logic 400, however, receives an n-bit decrementerror signal DEC_CTRL<N:0> from the PVT sensing logic 300, then each bitof the decrement error signal is connected to one of the NMOStransistors 410, 412 and 414 and some of the NMOS transistors 410, 412and 414 are selectively turned ON based on the value of bits of theDEC_CTRL<N:0>. The turned ON NMOS transistors decrease the output drivesignal, which is supplied to O/P driver of the transistors. Asdiscussed, the output drive signal changes the value of the output drivevoltage and an output drive current of the transmitter. The embodimentreduces the total output drive current flowing to the O/P driver of thetransistor, and hence compensates for all the non-linearity errorsinduced due to PVT variations.

Therefore, the PVT correction logic 400 compensates for a decrease inthe value of applicable PVT parameters from the nominal value byincreasing the total output drive current flowing to the O/P driver ofthe transmitter. Conversely, the PVT correction logic 400 decreases thetotal output drive current flowing to the O/P driver of the transmitterto compensate for an increase in PVT parameters from the nominal value.

FIG. 5 is a flowchart of a method 500 for minimizing effects of on-chipPVT variations on performance parameters of a transmitter employing theadaptive circuit 200, as discussed in connection with FIG. 2. At step502, the adaptive circuit 200 generates a clock output signal from areference bias voltage using the oscillator 202. The counter 204, asdiscussed in connection with the adaptive circuit 200, counts the clockoutput signal to generate a count value at step 504. At step 506 theadaptive circuit 200 compares the count value with a reference countvalue to compute an error value corresponding to PVT variations,employing the comparison module 208. Finally, at step 508 the correctionlogic 210 adjusts an output drive signal of the transmitter to reducenon-linearity errors based on the error signal generated at step 506.

Method 500 can be performed in two phases: a simulation phase and anon-chip calibration phase. The simulation phase includes computingreference count value for different PVT corners, whereas the on-chipcalibration includes computing a count value corresponding to value ofPVT parameters at any point of time during on-chip operations. Theon-chip calibration phase also includes comparing the reference countvalue with the count value generated during on-chip operation to sensedeviations induced due to PVT variations. The simulation phase isdiscussed in connection with FIG. 6 and the on-chip calibration phase isset out in the following sections in connection with FIG. 7.

FIG. 6 is a flowchart depicting the simulation phase of a method 600 forminimizing non-linearity errors induced in output voltage response of atransmitter. It will be understood by those skilled in the art that thesimulation phase is conducted on a simulation tool, during the design ofan IC, as discussed above. Thus, in the following discussion, all systemelements are simulated and are not the actual elements shown in, forexample, FIG. 2.

The simulation phase includes computing reference count values fordifferent PVT corners. At step 602, an adaptive circuit measures thefrequency of a VCO at nominal corner. At step 604, the adaptive circuitcalculates pulse width of a gated clock signal according to the value ofthe VCO frequency at nominal corner, as discussed in connection with aclock gate. The clock gate, as already discussed, generates a gatedclock signal using the pulse width computed at step 604, and this gatedclock signal is then supplied to an n-bit counter and a latch circuitfor counting the clock output of the VCO. The embodiment computes thepulse width using the following formula:clock gate pulse width=(2^(N))/2*VCO frequency@nominal corner whereN=number of counter bits  (1)

It is well known to those skilled in the art that an IC's performancevaries with performance parameters, and the performance of the IC mayvary from the best case to the worst case with PVT parameters varyingfrom best case corner to the worst case corner respectively. Forexample, an IC's output current corresponding to the best case PVTcorner is higher than the IC's output current corresponding to the worstcase PVT corner. A nominal corner, however, exists for PVT parametersthat result in an optimized output current for any IC. Thus, an IC issimulated under all PVT corners to characterize and verify theperformance of the IC. At step 606, the adaptive circuit computesreference count values for different PVT corners. Next, at step 608 allthe reference count values corresponding to different PVT corners arestored in a look-up table. These reference count values are employed asa reference to determine the PVT condition at any point of time duringon-chip operation. In an exemplary embodiment, a 4-bit counter maygenerate a count value of 1000 for nominal corner, 1110 for best casecorner, and 0100 for worst case corner.

FIG. 7 is a flowchart depicting the on-chip calibration phase of amethod 700 for minimizing non-linearity errors induced in a transmitter.During on-chip calibration phase, also referred to as post fabricationphase, an adaptive circuit operates to generate a count valuecorresponding to PVT conditions at any point of time and compensates forall deviations in the PVT conditions from their nominal values. Asdiscussed, with variations in PVT parameters the count value generatedusing the adaptive circuit also varies.

During the on-chip calibration operation, at step 702 the adaptivecircuit enables a counter and latch circuit to generate a count valuecorresponding to PVT condition at any point of time. The adaptivecircuit provides the gated clock signal generated at step 604 (FIG. 6)to the counter, and the latch circuit is preset to a known initial statecount value corresponding to the nominal corner, as computed duringsimulation phase. At step 704, the adaptive circuit subtracts thereference value corresponding to the nominal PVT corner computed at step608 (FIG. 6) from the count value generated by the counter. Thesubtraction of the reference count value from the count value results inan error count value and a sign bit value. Next at 706, the adaptivecircuit checks whether the error count value is positive or negative,i.e., the adaptive circuit verifies whether the sign bit value is apositive value or a negative value. If the sign bit generated ispositive, then at step 708, the adaptive circuit generates an n-bitdecrement error value, referred to as DEC_CTRL<N:0>. If the sign bitgenerated is negative, however, then at step 710, the adaptive circuitgenerates an n-bit increment error value, referred to as INC_CTRL<N:0>.

Thus, the invention dynamically senses and corrects all non-linearityerrors induced in output voltage response of a SerDes transmitter due toon-chip PVT variations, making the SerDes transmitter immune to PVTvariations.

It should be noted that the materials set out above constitutefunctional descriptions of the various embodiments of methods of theclaimed invention. Those of skill in the art will be able to put thosedescriptions into concrete form, using a variety of computer languages,coding languages, methods and tools. No best method for performing thisprocess is known, and it is believed that any of the well-known methodsand formats available to the art will suffice to produce operatingexamples of systems embodying the claimed invention.

Thus, it is apparent that there has been provided, with variousembodiments of the invention, a methodology for sensing and correctingPVT variations induced into a transmitter, which results in non-linearoutput voltage response of the transmitter. Although the invention hasbeen described in detail, it should be understood that various changes,substitutions, and alterations may be readily ascertainable by thoseskilled in the art and such alterations may be made without departingfrom the spirit and scope of the invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A circuit for minimizing non-linearity errors induced in output drivevoltage of a transmitter circuit due to on-chip process, voltage andtemperature (PVT) variations, comprising: an oscillator configured toconvert an input reference bias voltage into a clock output, wherein theinput reference bias voltage varies in response to PVT variations, andfrequency of the clock output varies according to changes in the inputreference bias voltage; a counter configured to receive input from theoscillator and generate a count value corresponding to the clock outputof the oscillator; a comparison module configured to generate an errorsignal, the error signal is calculated by comparing the count value witha pre-simulated count value; and a correction logic configured to adjustan output drive signal of the transmitter circuit based on the errorsignal.
 2. The circuit of claim 1, wherein the comparison moduleincludes: a look-up table containing a set of pre-simulated countvalues; a subtract module configured to subtract the pre-simulated countvalue from the count value; and a set of multiplexers configured togenerate the error signal.
 3. The circuit of claim 2, wherein the errorsignal is an increment error signal upon a determination that the countvalue is a positive value, and the error signal is a decrement errorsignal upon a determination that the count value is a negative value. 4.The circuit of claim 3, wherein the correction logic is configured toincrease the output drive signal of the transmitter circuit upon adetermination that the error signal is an increment error signal.
 5. Thecircuit of claim 3, wherein the correction logic is configured todecrease the output drive signal of the transmitter circuit upon adetermination that the error signal is a decrement error signal.
 6. Thecircuit of claim 1, wherein the oscillator is a voltage controlledoscillator (VCO).
 7. The circuit of claim 1, wherein the counterincludes: an n-bit counter; and a latch, the latch preset to a knownpreloaded initial state count value during calibration phase.
 8. Thecircuit of claim 1, wherein the output drive signal adjusts at least oneof the output drive voltage and an output drive current of thetransmitter circuit.
 9. A method for minimizing non-linearity errorsinduced in output drive voltage of a transmitter due to on-chip process,voltage and temperature (PVT) variations, the method comprising:generating a clock output signal from an input reference bias voltage,frequency of the clock output signal varying in response to PVTvariations; counting the clock output signal to generate a count value;comparing the count value with a pre-simulated count value to generatean error signal; and adjusting an output drive signal of the transmittercircuit based on the generated error signal.
 10. The method of claim 9,wherein the pre-simulated count value is contained in a look-up table.11. The method of claim 9, wherein the error signal is an incrementerror signal upon a determination that the count value is less than thepre-simulated count value, and the error signal is a decrement errorsignal upon a determination that the count value is greater than thepre-simulated count value.
 12. The method of claim 11, wherein theadjusting step includes increasing the output drive signal of thetransmitter circuit upon a determination that the error signal is anincrement error signal.
 13. The method of claim 11, wherein theadjusting step includes decreasing the output drive signal of thetransmitter circuit upon a determination that the error signal is adecrement error signal.
 14. The method of claim 9, wherein the adjustingstep adjusts at least one of the output drive voltage and an outputdrive current of the transmitter circuit.